Q-METER(RLC METER)

Group membersNajeeb ullah Naveed khan

What is Q –Meter ?

• Q meter is design for measuring the Q factor of a coil and for measuring inductance, capacitance and resistance of an electric circuit at radio frequency. It is also known as RLC meter.

• A direct-reading instrument which measures the Q of an electric circuit at radio frequencies by determining the ratio of inductance to resistance, and which has also been developed to measure many other quantities. Also known as quality-factor meter.

Introduction • Inductors, capacitors and resisters which have to

operate at radio frequencies cannot be measured satisfactorily at lower frequencies.

• The resonance methods are employed in which th e unknown component may be tested at or near its normal operating frequency.

• We know that every inductor coil has a certain amount of resistance and the coil should have lowest possible resistance. The ratio of the inductive reactance to the effective resistance of the coil is called the quality factor or Q-factor of the coil.

Q = XL / R = ωL / R

Construction Q meter consist of the following major elements:• Variable calibrated capacitor • Variable –frequency ac voltage source • Coil to be tested or measured

All elements are connected in series . The capacitor voltage (Vc) and the source voltage ( E) are

monitored by voltmeters. One terminal of the coil is connected to the signal

generator (Ac voltage source) while the other terminal is connected to the variable capacitor.

Circuit diagram

Volt meter 1 shows the source voltage(E) Volt meter 2 shows voltage across variable capacitor Inductor and resister combine shows coil

Working principleThe operations of this useful laboratory instrument is based on the familiar characteristics of a series-resonant circuit, namely, that the voltage across the coil or the capacitor is equal to the applied voltage times the Q of the circuit. If a fixed voltage is applied to the circuit, a voltmeter across the capacitor can be calibrated to read Q directly.

VC = VL and

I=E/R

Operation• First of all The oscillator is set to the desired frequency • Then the tuning Capacitor is adjusted for maximum

value under this condition and if the voltage is kept constant the voltmeter connected across the capacitor may be calibrated to read the value of Q directly.

• This measures value of Q in commonly regarded as the Q of the coil under test.

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Operation

• There is an error. The measured value of Q is the Q of the whole circuit and not of the coil.

• Thus the measured value of Q is smaller than the true value.

• Coils of high resistance are being measured the difference between the two value may be negligible but when measurements are done on low resistance coils, the error caused on this account may be serious.

VC= VL and I = E/R Q =ωL/R =1/ωCR Q=VL/E =VC/E

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